Screen printing stencil composition with improved water resistance

ABSTRACT

The invention resides in a photosensitive composition for screen printing. Preferably, the composition of the invention comprises a polymer comprising a vinylpyridine monomer or polymer having a pyridinium salt character and a photoreactable constituent. The resulting stencil, once dried, exposed, cured, and subjected to alkaline treatment obtains enhanced solvent/water resistance but can be reclaimed using an acidic wash. In use, the stencil provides a screen/stencil assembly or blockout utility of high structural integrity through repeated use and screen printing images of high resolution and quality.

This is a divisional of application Ser. No. 08/147,381, filed Oct. 26,1993, which application(s) are incorporated herein by reference.

FIELD OF THE INVENTION

The invention relates to polymeric preparations that can be used inscreen printing processes. Further, the invention relates to a film or aliquid polymeric composition that when used in combination with aqueousalkaline liquids, such as an alkaline ink, can form sharp long-livedstencil/screen assemblies used in screen printing operations. Moreparticularly, the invention relates to polyvinylpyridine (or pyridinium)containing films that can be cast from aqueous solutions which, afterimaging and in contact with an alkaline liquid such as a screen printingink, at a pH greater than 7, forms screen stencils with increased orimproved water resistance that can be used in screen printing processes.The composition of the invention can be reclaimed from the screen usingan aqueous acidic reclaiming composition.

BACKGROUND OF THE INVENTION

Images have been formed by screen printing processes for many years. Inscreen printing, an image is formed on a substrate by positioning astencil comprising a given pattern in or on a finely divided meshscreen. An ink is then applied through the stencil/screen assembly ontothe substrate resulting in a complementary image on the substrate. Suchpatterned stencils have been formed on screen constructions using avariety of techniques. Both photosensitive (negative and positive image)and non-photosensitive image forming processes have been used. Thephotosensitive materials used in making emulsion or film products can besolvent based or can be aqueous based. In many industries, includingphotostencil industries, the trend is away from solvent based materialsto materials manufactured from or based on aqueous formulations. Manyphotosensitive film and liquid dispersions are now based on aqueousproducts. In a similar vein, both solvent based inks and solventcontaining screen wash compositions have been used in screen printingoperations. In response to demands of the market, vendors of screenprinting materials have introduced water based inks. The use of waterbased or aqueous inks can cause images on stencils, made from waterbased films or aqueous emulsions, to swell or deteriorate reducing theeffective lifetime of the stencil and image quality. Because these imagelayers formed from typically aqueous materials tend to interact withwater based inks and swell, soften or otherwise deteriorate, asubstantial need exists to improve the printing properties of materialsderived from aqueous based systems for printing with water based inks.

BRIEF DESCRIPTION OF THE INVENTION

We have found that the pH sensitivity of polymers having pendant unitscomprising a non-photosensitive nitrogen heterocycle, preferably apolymer comprising vinylpyridine (pyridinium), can be used to improveboth photosensitive and non-photosensitive aqueous compositions andfilms, such as aqueous resist materials, blockout formulations and handcut films, when used in screen printing with aqueous ink formulations.The screen printing stencil composition of the invention comprises aphotosensitive polymer or photoresist material and a non-photosensitivepolymer comprising pendant units comprising a nitrogen heterocycle suchas a vinylpyridine and an aqueous vehicle or carrier. The invention canalso be embodied in non-photosensitive compositions and methods.

Polymeric materials have been used in resist materials as bothphotosensitive and non-photosensitive components. Non-photosensitivecomponents act as stabilizers, film forming agents, vehicles, pigmentsetc. A large variety of vinyl polymers take this role includingpolystyrenics, polyvinyl alcohols, polyacrylics, polyvinyl acetates;etc. In this regard pyridinium materials have also been used. Examplesof photosensitive materials using pyridinium materials include thefollowing. Leubner et al., U.S. Pat. No. 2,811,510, teach the use oflight sensitive polymeric stilbazoles and salts thereof which arequaternized polyvinylpyridine materials having photopolymerizableunsaturated units. Takeda et al., U.S. Pat. No. 4,245,027, teach a lightsensitive image recording material that uses a compound which onexposure to actinic radiation is capable of complexing apolyvinylpyridine material or a compound which releases a precursorcapable of complexing polyvinylpyridine to form a pyridinium salt. TheTakeda et al. material requires an activating exposure to appropriatewavelengths of light. Chiang et al., Journal of Orqanic Chemistry, Vol.X, pp. 21-25 (1945) teach the light sensitivity of stilbazole nuclei.Ichimura et al., Journal of Polymer Science: Polymer Chemistry Edition,Vol. 18, pp. 891-890 (1980), teach the reaction of the homopolymer of2-chloroethyl vinyl ether and the copolymer of N-vinylpyrrolidone withstilbazole to form photosensitive polymers that can be used toimmobilize enzymes. Borden, Canadian Patent No. 901,370, teachesphotosensitive polymeric compositions containing vinyl styryl pyridiniummaterials insolubilized by the action of actinic radiation. Haas, U.S.Pat. No. 3,507,846, teaches methods for polymerizing vinylpyridine intopolyvinylpyridine polymers. Yamaoka et al., U.S. Pat. No. 4,515,886,teach photosensitive materials useful as photoresists in the manufactureof semiconductors, transistors and other electrically active componentsin aqueous systems for photosensitive compositions. Day et al., U.S.Pat. No. 4,477,552, teach a use of a small amount of anon-photosensitive polyvinylpyridine (less than about 1 wt-%) as astabilizer for water sensitive colloidal diazo-resin sensitizers. Thepyridinium materials neither directly participate in the resistformation photoreactions nor provide any substantial properties to thestencil other than the stability of the diazo.

In a stencil or film, when in contact with acid, the pyridine polymerconverts into a water soluble or dispersible pyridinium form. Anopposite process can be easily performed by action of base. In contactwith a base, the acid salt (quaternary) form of the polymer regeneratesoriginal polymer (in free amine form) that is inherently waterinsoluble. Such properties could be demonstrated with polyvinylpyridine(substituted or not in a homopolymer or copolymer form), quinolines,polyvinyl quinaldines, polyvinyl quinazolines and other polymers thathave a nitrogen atom in a main chain or in a backbone.

The success of the use of the acid-base properties or pHsolubility/sensitivity of the preferred non-photosensitive pyridiniummaterials in the resist to improve the image forming properties of theresist is a surprise. When contacted with acid, the pyridine nitrogenforms an amine salt. As shown below, the acid base reaction can be usedto convert the polymer from a soluble pyridinium salt to a waterinsoluble free amine (pyridine) moiety. The insoluble pyridiniummaterials used at a concentration of greater than about 5 wt % hardensand improves the image. ##STR1##

The reaction of a sufficient proportion of the pyridine moieties to formthe pyridinium form renders the polymer soluble or dispersible inaqueousmedia. When contacted with sufficient proportions of alkaline materials,the water soluble pyridinium salt is converted to the free amine form(pyridine) which is substantially water insoluble. We have found thatthe alkaline character of common water-based inks cooperates with thenitrogen heterocycle in the polymeric composition in the screen stencilto improve the finished stencil. In the preferred mode of the stencilcomposition, the polymer comprising repeating units with the nitrogenheterocycle pendant groups can have a degree of polymerization of about50 to 100,000. The polyvinylpyridine can be a homopolymer having amolecular weight ranging from about 10⁴ to 10⁶. The polymer comprisingvinylpyridine can be formulated with other vinyl polymers,photosensitive compounds or compositions, surfactants, co-solvents,dyes, etc. The polyvinylpyridine can also be used in blockoutformulations without a photosensitive material. These materials can beused in forming hand cut stencils from sheet stock comprising thepolymers of the invention. Such stencils are made by first cutting intoa sheet of the materials of the invention an image as desired. The cutsheet is applied to a screen and then contacted with an alkaline ink ina printing operation. The alkaline ink improves the hardness and waterresistance of the film/screen.

A method of using the stencil composition of the invention includesapplying or coating the photosensitive stencil composition on a-screen,drying the composition, exposing the stencil (selectively imagewise)composition to actinic energy, developing the image, optionally dryingthe image, treating the stencil with an alkaline solution (in blockoutor printing operations) or treating a dried stencil with an aqueous,alkaline ink and printing a substrate. The used stencil can be reclaimedby treating the stencil material with an acidic reclaim composition.

We have found that a variety of the problems related to stencils, masks,and photoresists used in screen printing can be solved by compoundingthe stencil with a heterocyclic non-photosensitive nitrogen containingpolymer such as a polyvinylpyridine. Using the pH sensitive solubilityof such polymers comprising a nitrogen heterocycle or a pendantvinylpyridine, a mask or stencil can be hardened or made water resistanton a screen. A minimal proportion of ink used in the printing processcan result in hardening the stencil containing the polymeric materialsof the invention. The ink rapidly hardens the preferred pyridinium(soluble moiety) to pyridine (insoluble moiety) at the useconcentrations of the material in the photoresist. The alkaline ink actsto insolubilize the polyvinylpyridine and to print. To make a previouslyimaged and developed stencil with increased water resistance which canbe used in screen printing operations, the alkaline ink can be used.When operations are completed, the screen can be contacted with an acid(i.e, an acidic reclaiming composition) and the polyvinylpyridinestencil composition can be reclaimed.

The polyvinylpyridine used in the formulations of the invention ispreferably in the form of a pyridine material wherein the heterocyclicnitrogen has been reacted with an acid to form a pyridinium salt. Suchpolymers are substantially water soluble or dispersible and can easilybe compounded with other materials in manufacture of stencils or maskson screen. The compounded stencil materials have increased waterresistance as the pH of the alkaline treatment (ink or other liquidmaterials in contact with the stencil) increases above about 7. Thesematerials can be rendered useful in a medium of pH>7. Preferably, thebase is provided by a basic screen printing ink.

The water resistance of aqueous based photoresist materials can beimproved using an aqueous polyvinylpyridine in a water solubledispersible form, when treated with an alkaline ink. The alkaline inkfurther insolubilizes the imaged composition comprising thepolyvinylpyridine material on the screen. The stencil image is madeincreasingly water resistant through the action of the base present inthe ink. When the useful life of the screen/stencil assembly is at anend, the polymeric materials used in forming the image can be reclaimedby contacting the image with aqueous acidic reclaiming composition.

DETAILED DISCUSSION OF THE INVENTION

The invention can comprise an aqueous screen printing mask or stencilcomposition which can be insolubilized by the action of actinic lightand having screen printing properties that can be further improved by acooperation between an alkaline ink and a polymer with pendant unitscomprising a nitrogen heterocycle preferably a polyvinyl pyridiniumcomposition, as well as methods of using the same, screens resultingfrom the use of this composition, and screen images resulting from themethods of the invention. Generally, the film of the invention or theaqueous stencil composition comprises a polymeric material such as apolyvinylpyridine constituent and a photosensitive material. The film ordispersion may also comprise a second vinyl polymer or copolymer, anacid source, and a photoreactive compound. The invention can also beused in a blockout composition or process or in a hand cut stencil. Whenused in blockout processes the polyvinylpyridine composition, insolution or suspension and optionally combined with othernon-photosensitive components, can be applied to stencil flaws or otherareas not covered by the stencil image. The applied material can becontacted with an alkaline material, insolubilizing thepolyvinylpyridine in the blocked out areas. The blocked stencil can beused in standard printing operations.

The non-photosensitive polymer comprising repeating units comprising apendant nitrogen heterocycle can be made from a monomer of the formula:##STR2## wherein the R₁, R₂ or R₃ are independently a lower alkyl and Ais a nitrogen heterocycle including an unsubstituted or substitutedpyridine moiety, a quinoline moiety, a quinazoline moiety, a quinaldinemoiety etc. The heterocycle substituents are non-photoreactive.

A. Polyvinylpyridine

The preferred heterocycle containing polymer comprises a polymer made ofa vinylpyridine. Suitable polyvinylpyridine polymer compositions whichare preferably employed in the present invention include polymerscontaining at least one repeating unit derived from a polymerizablemonomer selected from the group consisting of the vinylpyridinepolymeric or copolymeric materials of the general formulae (I, II orIII) described below. More specifically suitable pollrvinylpyridinesinclude homopolymers as shown in the formulae; copolymers of at leasttwo different polymerizable monomers selected from the group consistingof the vinylpyridine compounds and other compatible copolymerizablepyridine monomers; and copolymers of at least one polymerizable monomerselected from the group consisting of the vinylpyridine compoundsrepresented by the general formulae (I, II or III) described below andat least one other polymerizable monomer. Suitable additionpolyvinylpyridine compounds, (polymers or copolymers) which are used canbe represented by the general Formulae (IV or V) below. ##STR3##

These polymers (Formulae I, II or III) are generally alkaline andinsoluble in aqueous solutions at pH >7, and may be solubilized ordispersed in aqueous solution by a mild or strong, inorganic or organicacid. Examples of categories of acids that can be used include bothLewis acids and proton donor acids. Such acids can form a complex withthe amine or can donate a proton to the amine group, such as thepyridine nitrogen forming a pyridinium (pyridine salt). The pyridinenitrogens can be fully or partially neutralized. Examples of suitableacids include phosphoric acid, acetic acid, sulfuric acid, hydrochloricacid, etc.

The reaction product is a water soluble or dispersible quaternaryvinylpyridine polymer such as that shown in Formulae (IV, fullyneutralized or V, partially neutralized). ##STR4##

The resulting polymer comprises either a fully neutralizedpolyvinylpyridine or a partially neutralized polyvinylpyridine, whereinn is 50 to 100,000, having an aromatic nitrogen with an amine saltcharacter which may be referred to as a pyridinium salt, the salt anionbeing a chloride or sulfate, for example. To promote solubility, theacid neutralization is generally effective to neutralize at least about30 wt-% of the polymer. As a result, neutralization may lead to asolution or dispersion of the polymer where the compound is completelyor, only partially soluble.

While not preferred, the pyridine polymers useful in the invention maybe substituted on the polymer backbone or on the aromatic nucleus. Insuch a case, non-photoactive substituent groups (R₁, R₂, R₃, or R₄) ateach of the positions may independently comprise a hydrogen atom or astraight or a branched chain alkyl group containing 1 to 7 carbon atoms(e.g., a methyl group, an ethyl group, an n-propyl group, an isopropylgroup, an n-butyl group, a t-butyl group, an n-heptyl group, etc.).

Specific examples of such substituents include a hydrogen atom, a methylgroup, an ethyl group, an n-propyl group, an isopropyl group and ann-butyl group.

Preferred specific examples of vinylpyridine monomers include4-vinylpyridine, 3-vinylpyridine, 2-vinylpyridine,2-methyl-4-vinylpyridine, 2-ethyl-5-vinylpyridine,3-methyl-5-vinylpyridine, 3-n-butyl-5-vinylpyridine,2-ethyl-4-vinylpyridine 3-methyl-4-vinylpyridine,3-isopropyl-4-vinylpyridine, 2-methyl-5-vinylpyridine, and the like.Generally, preferred polymers will have a molecular weight rangingpreferably from about 10⁴ to 10⁶.

B. Copolymers

The screen printing stencil composition of the invention may comprisethe vinylpyridine structures shown in Formula I through V.Alternatively, the screen printing stencil composition of the inventionmay comprise a copolymer of the structures shown in Formula I through Vand one or more comonomers.

In the case of copolymers, the copolymer may be a copolymer of two ormore of the above-described vinylpyridine monomers or the copolymer maybe a copolymer of one or more of the above-described vinylpyridinemonomers and one or more other addition polymerizable monomers capableof copolymerizing with the above-described vinylpyridine monomers. Abroad range of different copolymerizable monomers can be used. There isno restriction on the copolymerization ratio between vinylpyridine incopolymers containing only vinylpyridine monomers but in copolymers withother copolymerizable addition polymerizable monomers the mole fractionof the vinylpyridine monomer units to the total monomer units in theresulting copolymer is preferably more than about 0.3. Further, theresulting copolymer must be insolubilized due to the interaction of thepyridine moiety with the aqueous alkaline ink.

Examples of suitable addition polymerizable monomers capable ofcopolymerizing with the vinylpyridine polymers of the general formulae(I through V) above which can be used include mono-olefin compounds suchas ethylene, propylene, butene, etc.; diene compounds such as butadiene,isoprene, chloroprene, methoxybutadiene, cyclopentadiene,1,3-cyclohexadiene, etc.; vinyl halide compounds such as vinyl chloride,vinyl bromide, vinyl fluoride, etc.; halopropene compounds such as2-chloropropene, 1-chloropropene, 2-fluoropropene, etc.; vinyl ethercompounds such as vinyl methyl ether, vinyl ethyl ether, vinyl n-propylether, vinyl isopropyl ether, vinyl n-butyl ether, vinyl isobutyl ether,vinyl tert-butyl ether, vinyl neopentyl ether, vinyl 2-methoxyethylether, vinyl 2-chloroethyl ether vinyl 2,2,2-trifluoroethyl ether, vinylbenzyl ether, etc.; styrene compounds such as styrene, α-methylstyrene,4-chlorostyrene, dichlorostyrene, 2,5-dimethoxystyrene, 4-methylstyrene,4-ethylstyrene, etc.; aromatic vinyl compounds other than styrene, etc.;N-heterovinyl compounds other than the compounds represented by formulae(I) and (II) such as N-vinylcarbazole, N-vinylpyrrolidone, etc.; acrylicacid compounds such as acrylic acid, methacrylic acid, etc.; acrylicacid ester compounds such as methyl acrylate, ethyl acrylate, butylacrylate, etc.; methacrylic acid esters such as methyl methacrylate,ethyl methacrylate, n-propyl methacrylate, n-butyl methacrylate,isobutyl, isobutyl methacrylate, n-hexyl methacrylate, 2-ethylbutylmethacrylate, n-lauryl hexyl methacrylate, 2-ethylbutyl methacrylate,n-lauryl methacrylate, 4-(tert-butyl)phenyl methacrylate, bornylmethacrylate, etc.; acrylonitrile compounds such as acrylonitrile,methacrylonitrile, etc.; acrylamide compounds such as acrylamide,N,N-dimethylacrylamide, N-(1,1-dimethyl-3-oxobutyl)acrylamide, etc., andthe methacrylamides corresponding thereto; vinyl methyl ketone; methylisopropenyl ketone; maleic anhydride; vinyl ester compounds such asvinyl acetate, vinyl chloroacetate, vinyl n-butyrate.

C. Aqueous Vehicle

Generally, the screen stencil composition of the invention is producedas an aqueous composition. The compositions of the invention can be madeby dispersing or dissolving the components in water or a water blend.Preferably, the materials of the invention are made by dissolving orsuspending the polyvinylpyridine polymer composition in an aqueousphotoresist dispersion. The films of the invention can be made from suchan aqueous material. For the purposes of this invention, the vehicle isthe liquid continuous phase of the composition and can contain a majorportion of water and other miscible co-solvents. The vehicle may alsocontain other conventional ingredients including dyes, wetting agentsand may also comprise any number of other constituents. The aqueousvehicle is formulated to dry quickly and have a viscosity permittingease of application. The polyvinyl polymer is suspended or emulsified inthe aqueous phase. Generally, any liquid which will provide thesefunctions without creating a volatile organic character in thecomposition may be used in accordance with the invention.

Various carriers include water, mono-, di-, and polyhydric alcohols suchas methanol, ethanol, n-propanol and isopropanol, alkene and alkeneglycols, and the like.

D. Photoreactive Compound

The stencil composition of the invention comprises a photosensitivecompound to cure the stencil once it is placed on the screen. Thepreferred compound will be aqueous soluble and photoreactive whenexposed to an actinic source of energy. The photosensitizing additivesto be used in this invention may be low or high molecular weightsubstances.

Sensitizers commonly used in photopolymerizable resist compositions aremonomers having photosensitive ethylenically unsaturated groups such asvinyl, acryloyl, methacryloyl, allyl, vinyl ether, acrylamide, or othergroups or prepolymers thereof having an average degree of unsaturationof about 1 to 5. Examples of sensitizers having a single ethylenicallyunsaturated group include acrylamide, acrylic acid, methacrylic acid,methyl methacrylate, and methylol acrylamide. Preferred polyfunctionalsensitizers have two or more photosensitive ethylenically unsaturatedgroups including sensitizers such as pentaerythritol triacrylate,pentaerythritol tetraacrylate, pentaerythritol trimethacrylate,pentaerythritol tetramethacrylate, 2,2,-dibromo methyl, 1,3-propanediacrylate, triallyl isocyanurate, N,N'-methylene bis-acrylamide, andprepolymers thereof. The most preferred sensitizer comprisespentaerythritol triacrylate.

The composition of the present invention can further comprise a diazoresin complex photocross-linking agent. As the diazo resin,water-soluble diazo resins formed by condensing diazotization productsof diphenylamines such as p-aminodiphenylamine,4-amino-4'-methyldiphenylamine, 4-amino-4'-ethyldiphenylamine,4-amino-4'-methoxydiphenylamine, 4-amine-4'-chlorodiphenylamine and4-amino-4'-nitrodiphenylamine with aldehydes such as p-formaldehyde,acetaldehyde, propionaldehyde and n-butylaldehyde can be used. Thecomplexes can be formed between the above-discussed condensationproducts and sulfuric acid salts, phosphoric acid salts and zincchloride double salts. The photocross-linking agent can be used in anamount of up to 20 parts per 100 parts of the light-sensitiveheterocyclic quaternary group-added polymer.

Polymerization reaction initiators suitable for use in the invention arethose materials which will easily generate free radicals upon exposureto heat or ultraviolet radiation. Examples of these initiatcrs includebenzoinalkyl ethers, Michler's ketone, ditertiary butyl peroxide,dibenzothiazollyl-disulfide, dibromoacetophenone, anthraquinone,azobisisobutyronitrile, and the like. In a similar fashion theethylenically unsaturated resists of this invention can contain optionalfunctional compounds such as defoamers, surfactants, dyes, antioxidants,or other additives commonly used in this art.

Additional optional additives that can be used in the practice of thisinvention include surfactants used to increase the compatibility of theaqueous resist composition with the surface of the object and to promotean even coating. A defoamer can be used to insure that the layer ofresist is free of small bubbles and other foamed species. A dye can beused to permit visualization of the position and approximate thicknessof the resist composition. An antioxidant can be used for thepreservation of the concentrations of the crosslinking activator.

The photoresist film comprising the photopolymer may be formed in anymanner known to those skilled in the art including spraying, casting,reverse or roll coating, dipping, and doctor blading. Other methods offormation and photosensitive uses beyond those specifically disclosedherein will be recognized by the practitioner. These methods and uses ofthe invention are intended to be within the scope of the claims appendedto the end of this specification.

There is no specific limitation for the amount of addition of thesesubstances, but a preferable range is from about 10% to 50% by weightmore preferably from about 10 wt-% to 30 wt-%, and most preferably fromabout 15 wt-% to 25 wt-%.

E. Vinyl Polymers

The aqueous photosensitive materials of the invention can contain avinyl polymer in solution or dispersion form to aid in forming theimages of the invention. Such vinyl polymers are film forming materialscommonly used in aqueous solution or suspension.

In the case of copolymers, the copolymer may be a copolymer of two ormore of the monomers disclosed below. There is no restriction on thecopolymerization ratio between monomers. Further, the resultingcopolymer must be soluble or dispersible in the aqueous resist.

Examples of suitable addition polymerizable monomers capable ofcopolymerizing with the vinylpyridine polymers of the general formulae(I through V) above which can be used include mono-olefin compounds suchas ethylene, propylene, butene, etc.; diene compounds such as butadiene,isoprene, chloroprene, methoxybutadiene, cyclopentadiene,1,3-cylcohexadiene, etc.; vinyl halide compounds such as vinyl chloride,vinyl bromide, vinyl fluoride, etc.; halopropene compounds such as2-chloropropene, 1-chloropropene, 2-fluoropropene, etc.; vinyl ethercompounds such as vinyl methyl ether, vinyl ethyl ether, vinyl n-propylether, vinyl isopropyl ether, vinyl n-butyl ether, vinyl isobutyl ether,vinyl tert-butyl ether, vinyl neopentyl ether vinyl 2-methoxyethylether, vinyl 2-chloroethyl ether vinyl 2,2,2-trifluoroethyl ether, vinylbenzyl ether, etc.; styrene compounds such as styrene, α-methylstyrne,4-chlorostyrne, dichlorostyrene, 2,5-dimethoxystyrene, 4-methylstyrne,4-ethylstyrne, etc.; aromatic vinyl compounds other than styrene, etc.;N-heterovinyl compounds other than the compounds represented by formulae(I) and (II) such as N-vinylcarbazole, N-vinylpyrrolidone,N-vinylpyridine, etc.; acrylic acid compounds such as acrylic acid,methacrylic acid, etc.; acrylic acid ester compounds such as methylacrylate, ethyl acrylate, butyl acrylate, etc.; methacrylic acid esterssuch as methyl methacrylate, ethyl methacrylate, n-propyl methacrylate,n-butyl methacrylate, isobutyl, isobutyl methacrylate, n-hexyl1methacrylate, 2-ethylbutyl methacrylate, n-lauryl methacrylate,4-(tert-butyl)phenyl methacrylate, bornyl methacrylate, etc.;acrylonitrile compounds such as acrylonitrile, methacrylonitrile, etc.;acrylamide compounds such as acrylamide, N,N-dimethylacrylamide,N-(1,1-dimethyl-3-oxobutyl)acrylamide, etc., and the methacrylamidescorresponding thereto; vinyl methyl ketone; methyl isopropenyl ketone;maleic anhydride; vinyl ester compounds such as vinyl acetate, vinylchloroacetate, vinyl n-butyrate.

F. Acids

For the purposes of this invention, any organic or inorganic acid can beused that promotes the solubility of the nitrogen heterocycle containingpolymer. Any of a large number of sources of acidity may be used tolower the pH of the aqueous solution used to maintain the solubility ofthe pyridinium salt prior to, and during deposition as well as reclaimthe stencil composition. Both a Lewis acid, an electron pair acceptor,or a proton donor acid can be used. For the purposes of this invention aproton donor acid can be represented by the formula H⁺ X⁻ wherein H⁺ isan aqueous dissociable proton and X⁻ is an anion. Such anions caninclude protonated anion such as H₂ PO₄ ⁻, HSO₄ ⁻, etc. Representativesources of acidity include inorganic and organic acids such as hydro-and hypo- chloric, and bromic acids, as well as, sulfuric acid,phosphoric acid, and nitrogen based acids among others. Organic acidsuseful in accordance with the invention include saturated andunsaturated aliphatic carboxylic acids including mono-, di-, andpolycarboxylic acids, acetic acid, fumaric acid, propionic acid, citricacid, malic acid, lactic acid and mixtures thereof.

Generally, to ensure the solubility of the pyridinium salt the pH of thestencil composition as well as the reclaim solution should be less thanabout 8, and preferably from about 2 to 7.

Blockout Product and Process

The unique solubility properties of polyvinylpyridine can be used inblockout processes and products. The blockout process involves forming acoating on a screen in a non-photosensitive process. Such processes areoften used to repair flaws in images and to form borders or other largeblockout areas required to successfully form an image on a substrate.The blockout processes are commonly practiced by applying a materialcontaining a polymeric film forming material to the screen and dryingthe polymeric material into an intact film. We have found that thepolyvinylpyridine materials of the invention can be used in suchblockout materials to increase the water resistance of blockoutproducts. Commonly a blockout product can comprise in a water base, afilm forming polymer, a polyvinylpyridinium material, and optionally,acid if needed to form this pyridinium acid salt. A blend of polymericmaterials can be used in the blockout material to optimize the filmforming properties. Blockout materials on the screen can be reclaimedusing an aqueous reclaiming acidic composition in the same reclaimingprocess as the image formed from the photosensitive materials. Any ofthe vinyl polymers having nitrogen heterocycle side groups can be usedin forming the blockout products of the invention. The polymeric filmforming compositions are all well known materials and are discussedabove in the vinyl polymers.

Inks

In accordance with the invention, the stencil may be used with anynumber of aqueous alkaline ink compositions. Preferred ink compositionsinclude those which are stable in an aqueous, alkaline median or carrierso as to not solubilize or otherwise detract from the developed stencilcomposition.

Useful inks include the Aquasoft 400 LF ink product series made byPANTONE, the SINVAQUA ink by SUMMIT, the 2500, 2700, 8500 and 9500 inkseries sold by NA2-DAR, and other well known aqueous alkaline inks. Thebasicity of the inks causes the pyridine moiety of become insoluble. Inblockout applications the polymer can be contacted with an alkalinesolution to render the polymer insoluble. The solution can be made inany suitable aqueous liquid, water being preferred. The useful alkalinematerials include any material that can produce an alkaline pH in theaqueous liquid. Conventional weak and strong bases can be used. Examplesof suitable bases include inorganic bases such as NaOH, KOH, NaCO₃,NaHCO₃, NH₄ OH, and organic bases such as monoethanol amine, diethanolamine, etc.

Screen Fabrication and Printing

The stencil composition of the invention may be used in any number ofapplications based upon screen printing. Non limiting examples includethe following process.

Generally, screen stencils can be prepared by either applying an aqueousemulsion, containing a photosensitive material and the heterocyclicgroup containing polymer or polyvinylpyridine, to a screen mesh.Alternatively, a stencil can be made by applying a photoreactive ornon-photoreactive water soluble film to the screen mesh.Non-photoreactive films can be imaged in a number of ways including handcutting, hand painting, etc. The film or emulsion can be dried ifnecessary prior to exposure. The stencil can be exposed to actinicradiation using either a photopositive or photonegative method totransfer the image. The portion of the stencil to be removed (dependingon the photopositive or photonegative nature of the exposure process) isdeveloped or washed out with a waterstream until the undesirable portionof the stencil is completely removed. The image stencil is then driedand then applied in printing operations using preferably an alkalineink. When contacted with an alkaline ink, the materials made from theaqueous emulsion obtained increase resistance to the effects of waterdue to the presence of the heterocyclic containing polymer or thepolyvinylpyridine material. In an alternative preparatory method, afterthe film is developed or washed out with a water spray, the screen,either wet or dry, can be treated with a basic aqueous solution tofurther insolubilize the stencil prior to printing operations. The basicaqueous solution can be applied to either a wet or dry screen. However,preferably the aqueous alkaline solution is applied directly to a wetscreen immediately after development. The non-photosensitive heterocyclecontaining polymers or polyvinylpyridine are more easily converted intoan insoluble form when exposed to the alkaline solution in an aqueousmedium. The alkaline aqueous solution is allowed to soak into the screenfor a period of time. The solution may be applied either by spraying thesolution directly on the screen or by the application of some materialdirectly onto the screen by wetting, blotting, dipping or immersing thescreen into a volume of the alkaline aqueous solution. After a period oftime sufficient to ensure that the non-photosensitive polymers aresufficiently converted into the free pyridine form, the alkaline aqueousmaterial can be washed from the screen with a water rinse. The stencilcan be dried and can be printed immediately with an alkaline water basedink.

WORKING EXAMPLES

The following working examples are provided to further illustrate thevarious characteristics and attributes of the invention. These workingexamples should not be construed as limiting but rather as merelyillustrative of the invention.

Comparative Preparation

Mixtures of the following components were made by agitation at 1000R.P.M. for a period of 25 minutes. The components were mixed in theorder they are listed.

    ______________________________________                                        Constituent      Wt-%                                                         ______________________________________                                        Polyvinyl alcohol                                                                              73.17                                                        Dye              0.06                                                         Polyvinyl acetate                                                                              21.53                                                        Surfactant       0.12                                                         Alcohol          0.71                                                         Water            4.05                                                         Diazo            0.36                                                         ______________________________________                                    

Example #1 represents a solvent/water resistant screen printing stencilpreparation in accordance with the invention.

Example 1

A portion of the polyvinyl alcohol in the comparative example issubstituted with polyvinylpyridine (25% active in a methanol solution)and phosphoric acid. The remaining ratios of components remain the sameensuring that the only change is substitution of polyvinyl alcoholthrough the addition of polyvinylpyridine.

    ______________________________________                                        Constituent      Wt-%                                                         ______________________________________                                        PolyvinlyPyridine                                                                              5.35                                                         H.sub.3 PO.sub.4 0.52                                                         Polyvinyl alcohol                                                                              67.30                                                        Polyvinyl acetate                                                                              21.53                                                        Dye              0.06                                                         Surfactant       0.21                                                         Alcohol          0.71                                                         Water            4.05                                                         Diazo            0.36                                                         ______________________________________                                    

Compositions described in example 1 and the comparative example wereapplied to a screen of 110 mesh made of polyester, dried, exposed to apattern of light and washed with water to obtain a clear negative image.Materials were then printed, side by side, with water-based ink (pH>8),alkaline. By the action of the printing with the ink, thepolyvinylpyridine salt, which was made soluble in water by the acid toensure addition to the emulsion, is converted back to its waterinsoluble form, thereby making the resist water-resistant. The materialswere printed until the failure of the comparative resist mixture. Theresist mode of Example 1 was producing sharp images, therefore provingthe water-resistance of the Example 1 mixture.

Example 2

The composition formulated in example 1 was applied to a screen of 110mesh made of polyester, dried, exposed to a pattern of light and washedwith water to obtain a clear negative image and dried. Water-basedalkaline ink was applied to one half of the imaged screen and the otherhalf was left uncovered. The ink was left on the stencils for a periodof one hour. After one hour the ink was cleaned off the screen with asuitable screen wash. The stencils were then tested by double rubs withwater. The treated area was more resistant to water than the non-treatedarea.

Examples 3A and 3B

The following examples are further examples of compatible systems forthe invention. These examples employ a light-sensitive styryl pyridiniumpolymer in place of the Polyvinyl alcohol/Diazo used in Examples 1-3.Also, the amount of polyvinylpyridine was increased to further maintainthe water resistance. The following describes more methods to prove/testthe water resistance.

Mixtures of the following components were made by agitation at 1000R.P.M. for a period of 25 minutes. Polyvinylpyridine was mixed withacetic acid and the final solution was added to the other components inthe order they are listed.

    ______________________________________                                        EXAMPLE             3A     3B                                                 ______________________________________                                        Acetic acid         0.99   0.99                                               Polyvinylpyridine   19.78  19.78                                              SPP-H-13            24.73  14.83                                              Polyvinyl acetate 320                                                                             54.39  64.27                                              Dye                 0.12   0.14                                               ______________________________________                                    

Example 4

The compositions from examples 3A and 3B were applied to a screen of 110mesh and exposed to a pattern of light and washed with water.

Each applied coating was divided in half and one half of each wasexposed to a spraying of a 10% solution in water of sodium carbonate.This was then allowed to stand for a period of 30 minutes. The screenswere then washed with water of between 800-1200 p.s.i. and it wasdiscovered that the screen portions which were treated with the sodiumcarbonate were more water resistant than the non-treated areas.

Example 5

This example demonstrates the action of acid on polyvinylpyridine withrespect to solubility and insolubility.

A preparation of polyvinylpyridine and stoichiometric amount of aceticacid was coated on polyester carrier. The coating was then allowed todry at room temperature. Next, the film was peeled off the carrier andplaced in a volume of water. The film was completely solubilized.

Next, the same film was further dried at a temperature of 60 degrees C.for one hour and the test was repeated. The film was thenwater-insoluble.

The acetic acid, which was used to make an active salt of the pyridine(quaternize the pyridine), can be driven off as an acetic acid vapor,and the polyvinylpyridine is made insoluble again.

Example 6

The composition described in examples 3A and 3B were applied to a screenof 110 mesh made of polyester that was exposed to a pattern of light andwashed with water. One screen was dried at room temperature and one wasdried at 60 degrees C. for one hour. The screens were exposed to a waterspray at 800-1200 p.s.i. with the screen coatings dried at 60 degrees C.showing more water resistance than the room temperature dried coatings.

Example 7

A non-photosensitive blockout composition that can be used toselectively cover portions of a screen or repair flaws in an image caninclude 23.27 wt-% of a 25 wt-% active methanolic solution ofpolyvinylpyridine, 1.09 wt-% of acetic acid, 46.54 wt-% of a polyvinylacetate, 11.64 wt-% of a polyvinyl alcohol and 17.45 wt-% water.

The above specification, examples and data provide a completedescription of the manufacture and use of the composition of theinvention. Since many embodiments of the invention can be made withoutdeparting from the spirit and scope of the invention, the inventionresides in the claims hereinafter appended.

We claim:
 1. A method of forming a screen printing assembly, comprising a screen and a stencil composition, said stencil composition comprising a photoreactive resist composition and at least 5 wt-% of a vinyl polymer having repeating units derived from a heterocyclic nitrogen compound polymer, wherein said vinyl polymer is aqueous insoluble at an alkaline pH, and a carrier, said method comprising the steps of:(a) applying said stencil composition to said screen, wherein a stencil composition is an emulsion that is imaged after the emulsion is applied to the screen; and (b) contacting the image with an aqueous alkaline composition to harden said stencil composition.
 2. The method of claim 1 wherein the alkaline composition is an alkaline screen printing ink.
 3. The method of claim 1 wherein the degree of polymerization of the vinyl polymer is 25 to 100,000.
 4. The method of claim 3 wherein said stencil composition is insolubilized by the action of actinic light.
 5. The method of claim 1 wherein prior to contact with the aqueous alkaline solution, the vinyl polymer comprises a polyvinyl pyridinium chloride, a polyvinyl pyridinium sulfate, a polyvinyl pyridinium acetate or mixtures thereof.
 6. The method of claim 4 wherein the vinyl polymer is polyvinylpyridine having a molecular weight of about 10⁴ to 10⁶.
 7. A screen printing assembly comprising a screen and the stencil composition resulting from the method of claim
 1. 8. The assembly of claim 7 wherein the stencil composition comprises a polymer comprising repeating units of a vinyl monomer selected from the group consisting of a mono olefin, a diene, a vinyl halide, a vinyl ether, a styrene, an acrylic acid or acid ester, an acrylonitrile, an acrylamide, a ketone, an anhydride, and mixtures thereof.
 9. The assembly of claim 7 wherein the stencil composition comprises a polymer comprising a polyvinyl acetate, polyvinyl alcohol or a poly(ethylene-co-vinyl acetate).
 10. The assembly of claim 5 wherein said polyvinyl pyridinium is a homopolymer.
 11. A screen printing assembly comprising a screen and the stencil composition resulting from the method of claim
 6. 12. The assembly of claim 11 wherein said pyridinium compound comprises a copolymer.
 13. A method of forming a screen printing assembly, comprising a screen and a hand-cut imaged stencil composition, said composition comprising at least 5 wt-% of a vinyl polymer having repeating units derived from a heterocyclic nitrogen compound polymer, wherein said vinyl polymer is aqueous insoluble at an alkaline pH, and a carrier, said method comprising the steps of:(a) applying said imaged stencil composition to said screen; and (b) contacting the image with an aqueous alkaline composition to harden said stencil composition. 